The invention relates to ceramic layer systems having at least two adjacent layers whose conductivities are markedly different. Within the scope of this invention, conductivity is to be understood as the property of a material that permits conduction processes under certain conditions. An example of this type of ceramic layer system is included in a specific embodiment of gas sensors, which will be described in detail below, for detecting gases present in small quantities in a surplus of other gases, for example oxygen, carbon monoxide, hydrogen and/or other combustible components in exhaust gases of internal combustion engines. In this case, the conductivities are related to oxygen ion conduction at higher temperatures. The gas sensors being used more frequently in the field are those whose essential elements include a first electrode that is exposed to the gas mixture, a second electrode that is also in contact with the gas mixture by way of a diffusion barrier, and a reference electrode. All of the electrodes are in contact with a material that is ion-conductive at higher temperatures, and are connected to each other in a conductive manner. Zircon dioxide has performed particularly well as an ion-conductive material. Built-in heating elements assure the temperatures at which ion conduction takes place, typically 450 to 800.degree. C. These temperatures are referred to hereinafter as the operating temperature of the gas sensor. The measured signals received by the electrodes indicate the concentrations of the gases to be detected, or changes thereto, and permit control of the operating state of the engine.
The cited components of the gas sensors are mechanically sensitive, and are therefore disposed on a comparatively thick ceramic substrate layer, or embedded into such a substrate layer. The ceramic substrate layer can also be made of zircon dioxide. The advantage of this is that the ceramic substrate layer, which is generally produced from a ceramic substrate film, and the ion-conducting layer, which is generally printed onto the substrate, can be fixedly connected to one another during co-sintering, a stage of the production process, without a phase shift. In the event of temperature changes, therefore, no voltages occur that can have a negative impact on the integrity of the gas sensor. A disadvantage, however, is that the heating elements must be insulated against the ion-conductive ceramic substrate, which can require up to four additional work cycles. A further disadvantage lies in the relatively high cost of zircon dioxide, so the mechanical stability of the sensor is disproportionately high.
The two above-listed disadvantages are avoided if the ceramic substrate comprises a less expensive material that is non-ion-conductive (or practically non-ion-conductive) and therefore does not need to be insulated against the heating elements. Gas sensors of this type contain a ceramic layer system having two adjacent layers of different composition and whose conductivities for oxygen ions vary considerably. The substrate layer is non-ion-conductive (or practically non-ion-conductive), whereas the ion conductivity of the adjacent zircon dioxide layer is higher by several orders of magnitude at the operating temperatures. However, gas sensors that include the materials under practical consideration for the substrate layer have demonstrated that they cannot be sufficiently thermally stressed. In particular, it has been found that first microcracks, then visible, larger cracks form particularly during heating to operating temperatures which, of course, must take place quickly; these cracks soon lead to breakdowns of the gas sensor.